As an alternative or add-on to satellite-based positioning systems, positioning systems in which a present position of a terminal is estimated based on a list of communication nodes (such as for instance cellular base stations, access points or beacons) that can presently be heard by the terminal and a database that contains identifiers and positions of the communication nodes and/or models for the coverage areas for a number of communication nodes have gained recent interest.
Given that the positions of the heard communication node are known and distances from the terminal to the heard communication nodes can be estimated, the terminal's position can be estimated through triangulation. The distance between a terminal and a communication nodes may for instance be estimated based on the path loss (i.e. how much the signal attenuates between the terminal and the communication node) using a channel model or on timing (or round-trip timing) measurements (i.e. information expressing how long signals propagate between terminal and communication node).
Alternatively, if models for the communication node coverage areas are known, then a terminal may pick the coverage areas for the heard communication nodes and find the intersection of these coverage areas. The terminal can then be assigned a position that is, for instance, the center-of-mass of the intersection area. Similarly an error estimate may be given based on the size of the intersection.
A model for the coverage area of a communication node can for instance be determined by having one or more terminals that are capable of determining their position send reports with their present position and a list of communication nodes that can presently be heard, gathering, for each communication node, the terminal positions reported into a set of terminal positions and deriving, from this set of terminal positions, a model for the coverage area of the communication node.